Toric or toroidal type traction drive continuously variable transmissions have an input disc, an output disc and a plurality of traction rollers that tractionally engage the input and output discs. Power is transferred through the transmission by the traction drive train between the input discs, output discs and the traction rollers. The transmission ratio between the input discs and the output discs is determined by the angle of operation of the traction rollers relative to the input and output discs. The input discs and output discs are preferably arranged in pairs with the input discs being opposite ends and the output discs being in the center. The input power is connected with the input discs by a toric input shaft and the output power is taken from the output discs by a toric output shaft. The toric output shaft passes through the center of at least one of the input discs.
To accommodate the positioning of the toric output shaft a dual axis design is common. These systems employ a countershaft to connect the two input discs to the power output. The counter shaft is located radially outward of the input and output discs which requires that the transmission have a larger barrel size than required by the radius of the discs.
In order to transmit the power tractionally, the input discs are urged toward each other by mechanical or hydraulic means to establish the proper normal force between the input discs, output discs and the traction rollers and the operating angle of the traction rollers. The most recent designs of these transmissions use electro-hydraulic control systems to apply the normal forces required. These systems permit a more accurate control of the forces needed to establish the correct normal force and the force which establishes the positioning of the traction rollers. In the prior art transmissions of the single cavity designs, the forces applied to the input discs are absorbed or reacted by the transmission housing through rotating bearings. While this is a very effective structurally, it does reduce the overall efficiency of the transmission due to the power loss in the relative rotation of the bearing surfaces.
In order to establish a geared neutral condition, a summing planetary is commonly used. The summing planetary has one member connected with the toric input, one member connected with the toric output and a third member connected with the transmission output. Two types of summing planetary gear sets have been used.
The most commonly used is a simple planetary gear set which has a sun gear, a ring gear and a planetary carrier assembly that includes a plurality of pinion gears meshing with both the sun and ring gears. The toric input is connected with the carrier assembly, the toric output is connected with the sun gear and the transmission output is connected with the ring gear. Depending on the tooth ratio between the sun gear and the ring gear, the transmission output can be stationary (in neutral), rotated in a forward direction or rotated in a reverse direction.
One co-axial arrangement that has been used is shown in U.S. Pat. No. 5,607,372. The co-axial arrangement shown therein includes two sun gears and a carrier assembly having a plurality or interconnected pinion gears which mesh with both the sun gears. The carrier assembly is connected with the toric input, one sun gear is connected with the toric output and the other sun gear is connected with the transmission output which happens to be a planetary gear arrangement and a summing differential. The toric arrangement shown in the above mentioned patent eliminates the need for a counter shaft to provide the input drive to the toric input.